(1) Field of the Invention
The present invention relates in general to printing and, more particularly, to an apparatus for supplying printing presses with a fountain solution.
(2) Description of the Prior Art
In the lithographic printing process, a plate carries an etched image that is ink receptive and the ink on this image is transferred to a surface that may be on paper or some other material, thereby imparting the image to the surface. The fountain solution, which is primarily water, settles over the blank areas of the plate and renders those areas ink repellent.
The typical fountain solution includes a fountain etch concentrate, water, and from ten to thirty percent by volume of isopropyl alcohol. The fountain etch concentrate includes an acidic component, such as phosphoric acid or citric acid, buffering salts, and a water-soluble resin or gum, such as gum arabic or cellulose gum.
A dampening or moistening system transfers the fountain solution from an open tray containing the fountain solution to the plate cylinder of the lithographic printing press. In such systems, one of the problems is that the open tray permits evaporation of the isopropyl alcohol wetting agent in the fountain solution to the atmosphere or ambient air. The wetting factor changes very little above twenty volume percent isopropyl alcohol, however, it changes greatly at low percentages. Thus, precise alcohol control is necessary in order to reduce the alcohol content to the ten volume percent range or lower. Lower alcohol levels result in an improved ink to water balance, thereby permitting easier to print half-tones and solids. In addition, a lower level of wetting agent results in quicker drying times which permits faster press speeds. Finally, lower alcohol levels reduce the alcohol odor in the press room.
Systems have been developed for supplying a properly mixed fountain solution to the press automatically, but these systems have not always performed satisfactory.
Many of these systems have been simple mechanical metering systems, such as a float and shut-off valve. Since such systems are pre-set, it is difficult to monitor the metering system so one never knows if the fountain solution is properly mixed.
Another more sophisticated system operated on the electrical conductivity principle, with the amount of additive that is introduced into the water being determined by the electrical conductivity of the fountain solution that has been produced. This technique may produce misleading results when more than one chemical is added, particularly where each has some effect on the electrical conductivity of the solution.
A more successful approach has been to monitor the specific gravity of the fountain solution directly and attempt to maintain the proper specific gravity of the solution by adding alcohol to the fountain solution as the specific gravity of the fountain solution changes.
One particularly well-known such system is disclosed in U.S. Pat. No. 3,557,817 to Edwin H. Royce, entitled "Control for Mixing Fluids of Different Specific Gravity", issued Jan. 26, 1971. The Royce system included a test chamber having a hydrometer freely floating in a sample solution drawn from the mixing chamber. An electrical conductor is on the hydrometer which moves with the hydrometer relative to the surface of the sample solution and is operable to actuate a relay to open or close a solenoid control valve which regulates the flow of alcohol to the mixing chamber.
Another such apparatus, based on the same principle, is disclosed in U.S. Pat. No. 3,848,618 also to Edwin H. Royce, entitled "Apparatus for Mixing Fluids of Different Specific Gravity", issued Nov. 19, 1974. This control system includes a U-shaped tube, one leg containing controlled liquid having a desired specific gravity and the other liquid having a sample solution circulating therethrough. As specific gravity of the sample solution changes, the surface of the control liquid makes or breaks a contact with an electrode connected to the apparatus to eject the less dense liquid into the sample solution.
In addition to the importance of controlling the volume percent of isopropyl alcohol, printing problems may also occur when the fountain solution pH deviates significantly above or below the pH range of between 4.5 and 5.5. Water soluble resins or gums, such as gum arabic, are present in the fountain etch concentrate and function as hydrophylic (waterloving) polymers, thereby helping to maintain hydrophylicity and prevent ink pickup in a background area. Gum arabic, for example, demonstrates maximum absorption to the non-image area at approximately a pH value of 4.4. Since the isopropyl alcohol is neutral, it has been the past practice to premix the fountain etch concentrate with water in a batch mode, adjust the water-fountain etch concentrate solution to the proper pH, and then add the appropriate volume percent of isopropyl alcohol by one of the methods discussed above. The pH value of the fountain solution is then occasionally checked by litmus paper.
Certain disadvantages become apparent with the above methods. First, the mechanical metering methods have an accuracy of approximately 3 percent, which is adequate to maintain the volume percent of the isopropyl alcohol near twenty percent, but is not sufficiently accurate to maintain the alcohol volume percent in the preferred range of ten percent.
Still another disadvantage with the present systems is that the fountain solution etch concentrate-water solution has been premixed prior to use.
It has thus become desirable to develop an apparatus for mixing and supplying fountain solution to printing presses which will provide precise proportioning of the mixed liquid ingredients of the fountain solution with consistently repeatable results while at the same time eliminating the need for premixing the fountain etch concentrate with water by automatically maintaining the pH of the fountain solution at a prescribed limit.